During 1981, the last year for which nationwide records are available, 82,460 hectares with a production value of $159,300,000.00 were devoted to watermelon in the United States alone. Of these, 19,800 hectares (value $52,700,000.000) were in Florida. Records for Florida in 1984 indicate that 24,300 hectares were planted with a value of $62,100,000.00. In addition, watermelon is a major crop in other regions of the world.
Of all watermelon cultivars, those with seedless fruit are the most desirable and bring the highest prices. Seedless fruit, in general, receives greater consumer acceptance and demand. For example, the recent release of a single seedless grape cultivar `Red Flame` was responsible for a jump from 10th to 4th in consumer fresh fruit preference for grapes and stimulated an explosion of new acreage. Although seedless watermelon cultivars have been available for several years, technical difficulties in producing seed have kept the availability of planting stock low and the price of seed prohibitively high. Additionally, much variation in quality occurs between seedless watermelons.
The seedless condition in watermelon is almost always the result of the presence of three homologous complements per cell rather than the usual two. Cells with three homologous chromosomes are said to be triploid and are designated as 3x. The inability of the triploid zygote to develop normally into an embryo causes the absence of seeds in triploid plants. The abnormal embryo formation causes cessation of normal ovular development into a seed at an early stage. Typically, seedless watermelons contain small edible white ovules, similar to those in immature cucumbers.
Triploid seeds are produced by crossing diploid (2x) lines containing 22 chromosomes per cell with tetraploid (4x) lines containing 44 chromosomes per cell. This results in seeds that produce triploid (3x) plants with 33 chromosomes. Triploid plants are true F.sub.1 hybrids so their production depends on development of diploid and tetraploid parental lines (Wall [1960] Am. Soc. Hort. Sci. 76:577-588).
For large-scale commercial production of triploid seed, tetraploid and diploid parental lines are planted in mixed plots and allowed to cross pollinate. Triploid seed is produced only in melons on tetraploid plants that are fertilized with diploid pollen. Therefore, an adequate supply of diploid and tetraploid seed must be available to produce large mixed stands. All commercially grown seeded watermelons are diploid; therefore, lines for use as diploid parents are abundant. The major limitation to producing seedless watermelon lies in the difficulty associated with producing sufficient seed for the tetraploid (4x) parental lines which are eventually pollinated with a diploid (2x) to produce the seedless triploid (3x) seed.
Tetraploid lines are produced from diploid seedlings by application of colchicine. With either diploid or tetraploids, once a desirable cultivar is identified, the plant is self-pollinated in order to build up adequate seed. Diploid seed is easily produced by open pollination of pure stands of a given diploid cultivar. Tetraploid seed, however, has proven to be very difficult to produce in large, commercially useful, quantities. This is largely due to the fact that tetraploids exhibit a high degree of self-sterility. As a result of this self-sterility, very few melons develop in a field of tetraploid plants. Also, none or only a small number of seeds are usually produced in each self-pollinated melon. A productive cross will yield only 50-100 seeds per fruit. Therefore, ten or more years are typically required to increase seed of a new tetraploid line to commercially acceptable numbers.
This extremely slow accumulation of parental seed has been a major roadblock to the commercial production of triploid seed for seedless watermelon. In fact, this roadblock has been of such magnitude as to squelch development of a substantial seedless watermelon industry. For instance, triploid seedless watermelon seed now costs from $220.00 per pound to $600.00 per pound compared to $91.00 per pound for diploid hybrids and $10.00 per pound for popular open pollinated varieties. Overcoming the difficulty in producing seedless watermelon would revolutionize the watermelon industry. The invention disclosed here alleviates major technical difficulties in triploid seed production.
In addition to the inability to produce sufficient quantities of tetraploid parental seeds of acceptable quality, seedless watermelon farmers are constantly at risk of having their few high quality tetraploid strains become available to competitors. This can happen because, in a field of diploid and tetraploid plants, some of the tetraploid plants will self pollinate rather than be cross-pollinated by the diploid plants. Of course, cross-pollination results in fruit containing the desired triploid seed. Self-pollination of the tetraploid, however, results in a melon bearing the tetraploid seed. These melons with tetraploid seed are physically indistinguishable from the melons with the triploid seed. Therefore, 4x seed becomes mixed with 3x seed during seed production and this mixed seed is often sold to growers. Also it is possible that the same melon can contain both 3x and 4x seed. This can happen when one tetraploid flower is pollinated with pollen from both a 2x and a 4x plant.
As a result, the grower often obtains a mixed stand of 4x and 3x melons. For example, these melons are physically distinguishable from each other. The tetraploid melons are can be striped but the triploid melons can be plain. Usually growers sell the 4x melon at a much reduced price as a lower quality seeded variety. Consequently, the seeds for the desired, and difficult to produce, tetraploid line are made available to the public.
The invention disclosed here allows seed producers to rapidly produce tetraploid parental plants, while minimizing or eliminating the possibility of the production of unwanted tetraploid melons.
One step in the claimed process involves cloning tissue cultures of the tetraploid parental line. The few previous cloning efforts with respect to watermelons have primarily concentrated on the feasibility of cloning diploid and triploid cultivars. This prior research was done with the hope that the triploid plants could be successfully cloned and sold as viable transplants to farmers (Barnes et al. [1978] Cucurbit Growers Coop. 1:21-22; Barnes [1979] Sci. Hort. 11:223-227; Anghel and Rosu [1985] Rev. Roumaine de Biologie, Biologie Vegetale 30:43-55). However, these efforts have not successfully resulted in a method of commercially producing the seedless triploid cultivar. Also, plant growth and fruiting performance in the field has not been documented. Furthermore, previous researchers have not suggested ways to overcome problems associated with the destruction of the early growth phases as a result of misting. Misting was believed by some to be necessary to provide an intermediate environment between culture and the field in order to acclimate plants. Using modified cloning procedures as part of a novel process for producing tetraploidal parent strains, the disclosed invention overcomes the major obstacles which have prevented the development of the seedless watermelon industry. Also, by using cloned tetraploids it is possible to produce more uniform seedless melons. The disclosed process facilitates the rapid increase of tetraploid lines without the need for self-pollination.